Hi stacey!!
You are incredibly wonderful to come in search of advice for how to help a friend with an ED. Being someone who suffers from an ED, I know how difficult it is sometimes to keep friends.
Although telling her parents may seem like a good thing, and others on this board may disagree, I don't think it's what's best. Your friend needs to admidt this to herself and then her family on her own time. If she knows you may tell her parents she may be less likely to confide in you or anyone else. I know this is not something you are qualified to handle, so maybe you could encourage her to seek help, or to tell a parent or a loved one, a school advisor or a counselor. But she needs to do her recovery for herself, otherwise it's not going to work (I'm speaking from experience...)
I want to applaud you for wanting to help her! Go you! That is an amazing thing and shows a great amount of strength and love! It's not easy watching a friend you love hurt themselves, and even if she may not express it to you, she's thankful to have you around.
The only advice I can give you, besides encouraging her to tell someone, or seek help (or come to this board, it's a wonderful place for love and support...) is this...if at all possible, DON'T LEAVE HER. This will not help her. Her ED is not about her family or her friends or anything else...it's about her, and the issues she has going on inside of her.
Ok...now that I've rambled for hours...
NEVER GIVE UP
~sarah~